In recent years, with emerging distributed renewable power generation, such as photovoltaic power generation, there has been a renewed interest in dc power grids. It is envisaged that a mixture of traditional AC power grids and emerging dc power grids will become a new form of power gird for the future. For a DC power grid provided by an AC power source, the AC voltage has to be rectified into DC voltage. In the process, voltage at double the mains frequency (e.g. 100 Hz for a 50 Hz mains) may be present as ripple in the dc voltage link unless a very large capacitor is used to provide energy buffering between the AC power source and the loads of the DC voltage link. Like the AC power systems, the DC power systems have voltage regulation requirements. Also, the dc link voltage is subject to disturbances for various reasons, such as sudden changes of large loads and power mismatches between power supply and demand
For most power electronics applications that require AC/DC and DC/AC conversions, and some that require DC/DC conversions, there is an intermediate DC-link interface to facilitate the power conversion as illustrated in FIG. 1. These applications are generally applied at two levels: (1) utility (appliance) level, (e.g. lighting systems, electrical vehicle (EV) battery charging and motor drives) and (2) power generation level (e.g. wind turbines and photovoltaic micro-grid power systems). For both levels of applications, it is crucial to guarantee a stable and reliable operation of the DC-link to which one or several DC devices or down-stream converters are connected.
A large variation of the DC-link voltage will cause efficiency and performance degradation of its cascading converters and will significantly increase a system's voltage stress. See, L. Gu et al., “Means of eliminating electrolytic capacitor in AC/DC power supplies for LED lightings,” IEEE Trans. Power Electron., vol. 24, no. 5, pp. 1399-1408, (May 2009). It may also lead to other undesirable issues. For instance, the variation of the DC-link in LED applications can lead to light flicker, which is detrimental to human health. See for example, B. Lehman et al., “Proposing measures of flicker in the low frequencies for lighting applications,” IEEE Energy Conversion Congress and Exposition, (2011), pp. 2865-2872 and A. Wilkins et al., “LED lighting flicker and potential health concerns: IEEE standard PAR1789 update,” IEEE Energy Conversion Congress and Exposition, (2010), pp. 171-178. In electric vehicle (EV) battery charging applications, the ripple power can heat up the batteries, thereby reducing their lifetime. T. Shimizu and T. Fujita et al., “A unity power factor PWM rectifier with DC ripple compensation,” IEEE Trans. Ind. Electron., vol. 44, no. 4, pp. 447-455, (1997) and T. Shimizu and Y. Jin et al., “DC ripple current reduction on a single-phase PWM voltage-source rectifier,” IEEE Trans. Ind. Appl., vol. 36, no. 5, pp. 1419-1429, (2000). In photovoltaic (“PV”) applications, the variation can reduce the power efficiency of the PV panels as disclosed in P. T. Krein et al., “Minimum energy and capacitance requirements for single-phase inverters and rectifiers using a ripple port,” IEEE Trans. Power Electron., vol. 27, no. 11, pp. 4690-4698, (November 2012). See also, S. H Lee et al., “Mitigation of low frequency AC ripple in single-phase photovoltaic power conditioning systems,” J. power Electron., vol. 10, no. 3, pp. 328-333, (2010).
The voltage variation of the DC-link can be a steady-state oscillation or a transient disturbance. Essentially, these variations are caused by the instantaneous power difference between the total input power injected into and the total output power extracted from the DC-link. Major contributors to steady-state oscillation in the DC-link are the single-phase AC/DC or DC/AC systems, which inherently possess a double-line-frequency (100 Hz or 120 Hz) power difference between their AC input and DC output at steady state. S. Wang et al., “A flicker-free electrolytic capacitor-less AC-DC LED driver,” IEEE Trans. Power Electron., vol. 27, no. 11, pp. 4540-4548, (November 2012); G. R. Zhu et al., “Mitigation of low-frequency current ripple in fuel-cell inverter systems through waveform control,” IEEE Trans. Power Electron., vol. 28, no. 2, pp. 779-792, (February 2013); M. Su et al., “An active power-decoupling method for single-phase AC-DC converters,” IEEE Trans. Ind. Informatics, vol. 10, no. 1, pp. 461-468, (February 2014); and H. Li et al., “Active power decoupling for high-power single-phase PWM rectifiers,” IEEE Trans. Power Electron., vol. 28, no. 3, pp. 1308-1319, (March 2013). Other contributors to DC-link variations include the presence of AC side harmonics, load transient, etc.
The simplest and most conventional approach to improving the stability of the DC-link voltage is to insert one or more parallel capacitors C into the DC-link interface, as shown in FIG. 1. The capacitors are typically electrolytic or E-Cap ones due to their high energy density (space-saving) and cost effective nature. However, their relatively short lifetimes pose critical challenges to the lifetime and reliability of the overall system. With the increasingly stringent reliability requirement needed in power, aerospace, automotive, and general lighting systems, a more reliable method of stabilizing the DC-link without the use of E-Caps is required. In particular, large voltage ripple across electrolytic capacitors leads to large capacitor current ripple and thus internal resistive loss and temperature rise inside the electrolytic capacitor.
Recently, various solutions involving the use of small long-lifetime capacitors (e.g. film caps) to stabilize the DC-link voltage of such systems have been proposed. One such solution is to intentionally reduce the instantaneous input-and-output power difference by manipulating the input and/or output operation waveforms of the power electronics. See the L. Gu, et al. article, the G. R Zhu et al. article, B. Wang et al., “A method of reducing the peak-to-average ratio of LED current for electrolytic capacitor-less AC-DC drivers,” IEEE Trans. Power Electron., vol. 25, no. 3, pp. 592-601, (March 2010); X. Ruan et al., “Optimum injected current harmonics to minimize peak-to-average ratio of LED current for electrolytic capacitor-less AC-DC drivers,” IEEE Trans. Power Electron., vol. 26, no. 7, pp. 1820-1825, (July 2011); Feng Gao et al., “Indirect DC-link voltage control of two-stage single-phase PV inverter,” IEEE Energy Conversion Congress and Exposition, (2009), pp. 1166-1172 and Y. M. Chen et al., “DC-link capacitor selections for the single-phase grid-connected PV system,” 2009 International Conference on Power Electronics and Drive Systems (PEDS), (2009), pp. 72-77.
Methods based on this solution are cost effective, requiring only a change of the control, with no modification of the existing hardware. However, they are achieved at the expense of sacrificing the system's performance in terms of having a high power factor (PF) and good total harmonics distortion (THD), good voltage regulation and high efficiency. See, the L. Gu, et al., B. Wang, et al., X. Ruan, et al., Feng Gao, et al., Y. M Chan, et al. and G.-R. Zhu, et al. articles. Thus, this solution is only eligible for applications with less stringent regulatory requirements.
Another solution is to incorporate additional circuits with controls that decouple the input-and-output power difference of the power electronics, as separate ripple power components. These circuits act as separate energy storage components that do not require an E-cap. Methods based on this solution are considered “active-filter” based. Except for a small loss of power efficiency, these methods typically cause little degradation of the system's performance. See the T. Shimizu, et al. article; P. T. Krein, et al. article; A. C. Kyritsis et al., “A novel parallel active filter for current pulsation smoothing on single stage grid-connected AC-PV modules,” European Conf. on Power Elect. and Appl., (2007), pp. 1-10; W. Chen et al., “Elimination of an electrolytic capacitor in AC/DC Light-Emitting Diode (LED) driver with high input power factor and constant output current,” IEEE Trans. Power Electron., vol. 27, no. 3, pp. 1598-1607, (March 2012); W. Qi et al., “A novel active power decoupling single-phase PWM rectifier topology,” IEEE Applied Power Electronics Conference and Exposition, (2014), pp. 89-95; Y. Tang et al., “Decoupling of fluctuating power in single-phase systems through a symmetrical half-bridge circuit,” IEEE Applied Power Electronics Conference and Exposition, (2014), pp. 96-102; and
R. Wang et al., “A high power density single-phase PWM rectifier with active ripple energy storage,” IEEE Trans. Power Electron., vol. 26, no. 5, pp. 1430-1443, (May 2011). These energy storage devices have relatively longer lifetimes. Thus, the E-Cap can be eliminated and overall system reliability can be enhanced.
However, the need to incorporate additional circuits mandates that existing power electronics hardware in pre-existing installations must undergo modification and re-design. This significantly increases the overall system's cost at both manufacturers' and consumers' ends. For the power generation level of applications, modification of hardware on-line is generally prohibited.